1485700 - about:blank is differently displayed between homepage (new tab) and new windows

Status: RESOLVED DUPLICATE of bug 1422507

(Firefox :: Tabbed Browser, defect)

Description

[Iulia Cristescu, QA [:JuliaC] (away, please needinfo? [e-mail])]

[Affected versions]:
- 63.0a1 (2018-08-23)
- 62.0b19 build1 (20180820172315)

[Affected platforms]:
- Windows 10 x64
- Windows 7 x64	
- Ubuntu 16.04 x64
- macOS 10.13.4	

[Steps to reproduce]:
1. Launch Firefox
2. Go to about:preferences#home and set the "Homepage and new windows" to "Blank Page" 
3. Click the Homepage button 
4. Open a new window

[Expected result]:
- The Blank Page is successfully displayed in both Homepage and new window

[Actual result]:
- The Blank Page from the new window has a darker shade (light gray) than the one displayed in the Homepage (white) 
- See the following screenshot https://drive.google.com/file/d/16iWjpHdtKU7gRqEWF5IlrbawK7CcdIXz/view?usp=sharing

[Regression range]:
- I will provide the related details as soon as possible

[Additional notes]:
- The issue is most easily noticed on Windows and macOS platforms (less visible on Ubuntu)
- Opening a new tab will fix the situation
- The issue is noticeable on all default, light and dark themes

Comment 1

[:Gijs (he/him)]

I somewhat suspect this might have the same cause as this issue:
(In reply to :Gijs (he/him) from bug 1362774 comment #151)
> *** besides a pre-existing issue with the 'blank' attribute not being
> removed from the tab and the tab thus not rendering content that the user
> inserts into the empty, null-principal'd about:blank document manually via
> e.g. devtools or a bookmarklet. I'll file separately, doesn't seem urgent
> given the STR.

But I think I never filed the other bug...

Comment 2

[Iulia Cristescu, QA [:JuliaC] (away, please needinfo? [e-mail])]

Back with details regarding the regression window: 
- last good revision 5d86f27fe905246f8a3211da099f5e546315188b
- first bad revision 0d93c364b753c9ecde20ab8e961bba61a9bb894d
- pushlog https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=5d86f27fe905246f8a3211da099f5e546315188b&tochange=0d93c364b753c9ecde20ab8e961bba61a9bb894d
- Bug 1379587 is a possible regressor